Leucite is a crystalline potassium aluminosilicate that, in stable form, possesses a tetragonal configuration at room temperature. Tetragonal leucite, also known as "low leucite," has been employed as a reinforcing agent in feldspathic dental porcelains. Such dental porcelain materials are described in, for example, U.S. Pat. Nos. 4,604,366 and 4,798,536. Since tetragonal leucite possesses a high coefficient of thermal expansion, the resulting feldspathic porcelains comprising tetragonal leucite dispersed therein as a discontinuous phase have correspondingly high coefficients of thermal expansion. For example, the tetragonal leucite-containing feldspathic porcelain powder sold under the trademark Optec.TM. by Jeneric/Pentron Inc., Wallingford, Conn. can be used to provide a dental porcelain body possessing a coefficient of thermal expansion of about 18.0.times.10.sup.-6 /.degree. C. when measured at 25.degree. C. to 500.degree. C.
When tetragonal leucite is heated to about 625.degree. C., it changes to a cubic polymorph and exhibits a volume change of 1.2%. This transformation is reversible. Upon cooling, the cubic leucite crystals revert to the more stable tetragonal polymorph. In contrast to tetragonal leucite, the cubic form of leucite, known as "high leucite," which is otherwise unstable at room temperature, possesses a coefficient of thermal expansion of about 3.times.10.sup.-6 (when measured at 625.degree. to 900.degree. C.).
The prior art discussed below shows that porcelain (also sometimes referred to as a glass-ceramic) materials containing cubic leucite stabilized at room temperature exhibit lower thermal expansion compared to tetragonal containing glass-ceramics and porcelains.
Rouf et al. in "Crystallization of Glasses in the Primary Field of Leucite in the K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 System" in Trans. J. Brit. Ceram. Soc., 77:36-39 (1978) describe an isothermal heat treatment method of crystallizing cubic leucite in the high viscosity system of K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 for both powder and bulk samples which employs TiO.sub.2, ZrO.sub.2, and P.sub.2 O.sub.5 as catalysts. The Rouf et al. method employs the use of high temperatures and long time periods and relies on the presence of large amounts of K.sub.2 O (approximately 18 weight percent) in the starting glass composition to form cubic leucite as the only crystalline phase. Bulk samples of the porcelain produced by the method disclosed in Rouf et al. do not comprise cubic leucite substantially uniformly dispersed therein.
Hermansson et al. in "On the Crystallization of the Glassy Phase in Whitewares," Trans. J. Brit. Ceram. Soc., 77:32-35 (1978) similarly disclose a heat treatment method of crystallizing cubic leucite in the high viscosity system of K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2. Hermansson et al. disclose that high K.sub.2 O content, a long firing time and a low content of CaO (approximately 1 weight percent) are required to stabilize the cubic phase of leucite at room temperature.
Prasad et al. in "Crystallization of Cubic Leucite By Composition Additives," 19th Annual Session, American Association For Dental Research, (1990), and Hahn et al. in "Importance of the Glass Ceramic System K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 in Dental Porcelain," Ceramic Forum Int./Ber. Dt. Keram. Ges. 57 (1980) No. 9-10, pp.208-214 each describe cubic leucite porcelains produced by volume crystallization of glasses containing about 2 mole % of Cs.sub.2 O. These porcelains are processed by a conventional method that involves smelting glass compositions with Cs.sub.2 O as one of the components. The resulting glasses, aside form the presence of Cs.sub.2 O, are reasonably close in composition to high-strength feldspathic porcelains. The role of cesium is to stabilize the cubic leucite phase at ambient temperatures. The cubic leucite phase is formed during crystallization heat treatment following melting and is retained in the porcelains at room temperature. Although it is a simple and cost-effective method, it does not provide sufficient control of the distribution and morphology of the leucite phase. The resulting materials suffer at least from one or both of the following deficiencies: too refractory to be processed in a dental lab setting (i.e., the fusion temperature is .gtoreq.1200.degree. C.; and grain size and uniformity of the leucite phase dispersion are not adequate to produce sufficiently strong porcelain.
None of the aforementioned prior art methods disclose a surface crystallization process. It is well known in the art that it is extremely important that the thermal expansion coefficient of a dental porcelain closely match the thermal expansion coefficient of the metal or porcelain material with which it is in contact. Since the leucite-containing dental porcelains of the prior art generally possess high coefficients of thermal expansion, they cannot be employed in combination with materials possessing significantly lower thermal expansion coefficients.
Accordingly, there is a need to for a core porcelain having a thermal expansion compatible with a variety of materials in the fabrication of dental restorations. It is desirous that an easy and cost-effective method is used to produce porcelain for such use.